An MRI apparatus can obtain an image indicating physical and chemical properties of a subject by placing the subject in an imaging space in which a uniform magnetostatic field is formed and using a nuclear magnetic resonance phenomenon occurring when the subject is irradiated with a radio frequency pulse. This image is mainly for medical use.
The MRI apparatus includes a magnetic device for forming a uniform magnetostatic field. A higher resolution image can be obtained as the magnetostatic field is larger. There are three kinds of magnets, i.e., a permanent magnet, a normal-conducting magnet, and a superconducting magnet. Among magnetic fields which can be generated by those magnets, the largest magnetic field is generated by the superconducting magnet. Therefore, currently, a major MRI apparatus generates a magnetostatic field with the use of a superconducting magnet.
The superconducting magnet includes a superconducting wire as a winding for allowing a current to pass therethrough. When the superconducting wire is cooled to have a certain temperature or less peculiar to a superconducting material, a current can flow through the superconducting wire with no resistance. Accordingly, it is possible to generate a large magnetic field by causing a large current to flow therethrough. Thus, the superconducting magnet needs to include a cooling mechanism for cooling a winding portion to maintain a low temperature state.
As a method for cooling a superconducting magnet, there is widely used a liquid helium immersion cooling in which a winding portion is immersed and is cooled in liquid helium. There is also widely used conduction cooling in which a chiller and a bobbin in which a coil is provided are connected by a heat transfer member made of a good heat conductor such as pure aluminum or oxygen-free copper and a superconducting magnet is cooled by the chiller via the heat transfer member. An advantage of the conduction cooling is that liquid helium serving as a coolant is not necessary. Because drain on a liquid helium resource has attracted attention all over the world in recent years, the above advantage of the conduction cooling is valued.
As a method for performing the conduction cooling on a superconducting magnet, Patent Literature 1 discloses a method for providing a heat transfer path by connecting a good heat conductor made of copper or the like to a bobbin made of stainless steel and connecting the good heat conductor to a chiller.